Bottom Line:
A thin layer of silica coating provides high colloidal stability and biocompatibility, and a versatile surface functionality.We demonstrate that after surface pegylation, these silica-coated magneto-fluorescent supernanoparticles can be magnetically manipulated inside living cells while being optically tracked.Moreover, our silica-coated magneto-fluorescent supernanoparticles can also serve as an in vivo multi-photon and magnetic resonance dual-modal imaging probe.

ABSTRACTMagneto-fluorescent particles have been recognized as an emerging class of materials that exhibit great potential in advanced applications. However, synthesizing such magneto-fluorescent nanomaterials that simultaneously exhibit uniform and tunable sizes, high magnetic content loading, maximized fluorophore coverage at the surface and a versatile surface functionality has proven challenging. Here we report a simple approach for co-assembling magnetic nanoparticles with fluorescent quantum dots to form colloidal magneto-fluorescent supernanoparticles. Importantly, these supernanoparticles exhibit a superstructure consisting of a close-packed magnetic nanoparticle 'core', which is fully surrounded by a 'shell' of fluorescent quantum dots. A thin layer of silica coating provides high colloidal stability and biocompatibility, and a versatile surface functionality. We demonstrate that after surface pegylation, these silica-coated magneto-fluorescent supernanoparticles can be magnetically manipulated inside living cells while being optically tracked. Moreover, our silica-coated magneto-fluorescent supernanoparticles can also serve as an in vivo multi-photon and magnetic resonance dual-modal imaging probe.

Figure 3: Silica-coated CS-SPsa, A representative TEM image of silica-coated CS-SPs (silica-CS-SPs) with a thin layer thickness of ~ 10.6 ± 0.7 nm. Scale bar is 200 nm. Insets show the histogram of the particle diameter distribution of silica-CS-SPs with an average diameter of 100 ± 12 nm (top right) and a zoomed-in TEM image of one silica-CS-SP (bottom right). Scale bar in the inset is 50 nm. b, An image of dark-field scanning TEM (STEM) and elemental line scan from a silica-CS-SP show that, while maintaining the core-shell superstructure, a thin layer of silica shell is uniformly deposited onto the CS-SP’s surface. Scale bar is 50 nm. c, HD diameter and relative photoluminescence (relat. PL) intensity of silica-CS-SPs as a function of storage time. d, Magnetic characterization (magnetization versus magnetic field at 300K) shows the superparamagnetism of silica-CS-SPs. e, Left, epi fluorescence image of silica-CS-SPs on a glass substrate. Right, a representative photoluminescence time trace of a single silica-CS-SP.

Mentions:
With their combination of both magnetism and fluorescence, CS-SPs have potential in various applications and technological devices. However, feasible applications, especially in biological systems, often require additional surface functionality, and this can be significantly hindered with CS-SPs that have a PVP polymer surface. To overcome this challenge and improve biocompatibility and colloidal stability, we encapsulate CS-SPs (~ 80 nm) with a thin silica shell through a sol-gel process38, 39. TEM characterization shows that each CS-SP is uniformly coated with a 10.6 ± 0.7 nm thick silica shell (Fig. 3a). STEM line scan, element mapping and 3D TEM tomography show that the core-shell superstructure is preserved (Fig. 3b, Supplementary Fig. 6 and Supplementary Movie 3). DLS measurements reveal that the HD diameter dramatically decreases from ~ 130 nm to ~ 100 nm, similar to the TEM-determined size of ~100 nm (Fig. 3a and Supplementary Fig. 7). The decrease in HD size implies a complete removal of the bulky PVP layer. Importantly, these silica-coated CS-SPs (silica-CS-SPs) display a high degree of colloidal stability. No measureable changes in both the PL intensity and HD size can be observed after 6-month at 4 °C (Fig. 3c). These silica-CS-SPs exhibit superparamagnetism at room temperature with a saturation magnetization of 15.2 emu g−1 (1.4 × 10−14 emu per particle) (Fig. 3d and Supplementary Fig. 8 and Supplementary Table 3). The decreased magnetization compared to that of free Fe3O4 MNPs (63.7 emu g−1, Supplementary Fig. 9) is due to non-magnetic components inside each silica-CS-SP (the QDs, the silica layer, and the organic ligands) with a mass percentage of 75.2% (Supplementary Fig. 10 and Supplementary Table 3). The PL QY of the silica-CS-SPs was measured to be ~12% using a 405 nm excitation light, comparable with that of the PVP-coated CS-SPs (Supplementary Fig. 11). The decreased QY compared to free QDs (PL QY of 94%) is in large part a result of the MNPs strongly absorbing this wavelength light. Under the illumination of a continuous wave laser at a power density of 22 W cm−2 for ~ 2800 second, silica-CS-SPs did not show evidence either of blinking or photo-bleaching (Fig. 3e), thus making them an ideal tool for single particle tracking. To demonstrate the silica-layer enabled surface functionality, methoxy-polyethylene-glycol silane (mPEG-silane, MW5000) was conjugated to the CS-SPs (Supplementary Fig. 12). The resulting particles have nearly neutral surfaces (−5.1 ± 2.3 mV, Supplementary Fig. 13), minimal cell toxicity (Supplementary Fig. 14) and minimal protein-adsorption (Supplementary Fig. 15). These features allow for the ultimate use of these CS-SPs in various biological systems.

Figure 3: Silica-coated CS-SPsa, A representative TEM image of silica-coated CS-SPs (silica-CS-SPs) with a thin layer thickness of ~ 10.6 ± 0.7 nm. Scale bar is 200 nm. Insets show the histogram of the particle diameter distribution of silica-CS-SPs with an average diameter of 100 ± 12 nm (top right) and a zoomed-in TEM image of one silica-CS-SP (bottom right). Scale bar in the inset is 50 nm. b, An image of dark-field scanning TEM (STEM) and elemental line scan from a silica-CS-SP show that, while maintaining the core-shell superstructure, a thin layer of silica shell is uniformly deposited onto the CS-SP’s surface. Scale bar is 50 nm. c, HD diameter and relative photoluminescence (relat. PL) intensity of silica-CS-SPs as a function of storage time. d, Magnetic characterization (magnetization versus magnetic field at 300K) shows the superparamagnetism of silica-CS-SPs. e, Left, epi fluorescence image of silica-CS-SPs on a glass substrate. Right, a representative photoluminescence time trace of a single silica-CS-SP.

Mentions:
With their combination of both magnetism and fluorescence, CS-SPs have potential in various applications and technological devices. However, feasible applications, especially in biological systems, often require additional surface functionality, and this can be significantly hindered with CS-SPs that have a PVP polymer surface. To overcome this challenge and improve biocompatibility and colloidal stability, we encapsulate CS-SPs (~ 80 nm) with a thin silica shell through a sol-gel process38, 39. TEM characterization shows that each CS-SP is uniformly coated with a 10.6 ± 0.7 nm thick silica shell (Fig. 3a). STEM line scan, element mapping and 3D TEM tomography show that the core-shell superstructure is preserved (Fig. 3b, Supplementary Fig. 6 and Supplementary Movie 3). DLS measurements reveal that the HD diameter dramatically decreases from ~ 130 nm to ~ 100 nm, similar to the TEM-determined size of ~100 nm (Fig. 3a and Supplementary Fig. 7). The decrease in HD size implies a complete removal of the bulky PVP layer. Importantly, these silica-coated CS-SPs (silica-CS-SPs) display a high degree of colloidal stability. No measureable changes in both the PL intensity and HD size can be observed after 6-month at 4 °C (Fig. 3c). These silica-CS-SPs exhibit superparamagnetism at room temperature with a saturation magnetization of 15.2 emu g−1 (1.4 × 10−14 emu per particle) (Fig. 3d and Supplementary Fig. 8 and Supplementary Table 3). The decreased magnetization compared to that of free Fe3O4 MNPs (63.7 emu g−1, Supplementary Fig. 9) is due to non-magnetic components inside each silica-CS-SP (the QDs, the silica layer, and the organic ligands) with a mass percentage of 75.2% (Supplementary Fig. 10 and Supplementary Table 3). The PL QY of the silica-CS-SPs was measured to be ~12% using a 405 nm excitation light, comparable with that of the PVP-coated CS-SPs (Supplementary Fig. 11). The decreased QY compared to free QDs (PL QY of 94%) is in large part a result of the MNPs strongly absorbing this wavelength light. Under the illumination of a continuous wave laser at a power density of 22 W cm−2 for ~ 2800 second, silica-CS-SPs did not show evidence either of blinking or photo-bleaching (Fig. 3e), thus making them an ideal tool for single particle tracking. To demonstrate the silica-layer enabled surface functionality, methoxy-polyethylene-glycol silane (mPEG-silane, MW5000) was conjugated to the CS-SPs (Supplementary Fig. 12). The resulting particles have nearly neutral surfaces (−5.1 ± 2.3 mV, Supplementary Fig. 13), minimal cell toxicity (Supplementary Fig. 14) and minimal protein-adsorption (Supplementary Fig. 15). These features allow for the ultimate use of these CS-SPs in various biological systems.

Bottom Line:
A thin layer of silica coating provides high colloidal stability and biocompatibility, and a versatile surface functionality.We demonstrate that after surface pegylation, these silica-coated magneto-fluorescent supernanoparticles can be magnetically manipulated inside living cells while being optically tracked.Moreover, our silica-coated magneto-fluorescent supernanoparticles can also serve as an in vivo multi-photon and magnetic resonance dual-modal imaging probe.

ABSTRACTMagneto-fluorescent particles have been recognized as an emerging class of materials that exhibit great potential in advanced applications. However, synthesizing such magneto-fluorescent nanomaterials that simultaneously exhibit uniform and tunable sizes, high magnetic content loading, maximized fluorophore coverage at the surface and a versatile surface functionality has proven challenging. Here we report a simple approach for co-assembling magnetic nanoparticles with fluorescent quantum dots to form colloidal magneto-fluorescent supernanoparticles. Importantly, these supernanoparticles exhibit a superstructure consisting of a close-packed magnetic nanoparticle 'core', which is fully surrounded by a 'shell' of fluorescent quantum dots. A thin layer of silica coating provides high colloidal stability and biocompatibility, and a versatile surface functionality. We demonstrate that after surface pegylation, these silica-coated magneto-fluorescent supernanoparticles can be magnetically manipulated inside living cells while being optically tracked. Moreover, our silica-coated magneto-fluorescent supernanoparticles can also serve as an in vivo multi-photon and magnetic resonance dual-modal imaging probe.